@Alicia pointed out on Twitter that they only appear to be sending single riders when the regular queue in the Star Destroyer does get backed up. I then considered that with only one lane to ingest regular riders (versus the usual two), that line is far more likely to spill out into the hangar before incoming guests can be grouped. That does mean in theory single riders will be dispatched through the shuttle fairly regularly, but it also probably means both the standby and lightning lane queues will move more slowly prior to the merge. An interesting conundrum.